In the past, stacks of sealing elements have been used in packers to obtain a seal against the casing or tubing or wellbore. The packing elements are generally stacked next to each other and either abut each other or are spaced apart by booster sleeves or annular rings. As a result, the packing element in the center is hemmed in from above and below by an upper and lower element. One of the main problems with packing elements, especially where the radial distance to seal is significant, is the problem of extrusion. The central element in a three-element stack is generally less prone to extrusion because it is more fully supported from above and below and experiences similar support against extrusion from the upper and lower elements.
Many applications require insertion of a packer in a confined space where, in order to clear internal obstructions, it is important for the elements after the packer is released to return to their original shape as much as is possible. In that sense, the "memory" of the elements is significant because if the elements, once expanded for sealing, do not closely come back to their original shape, problems could ensue in trying to remove the packer. Some prior art packers have applied longitudinal tensile forces to get the elements to reassume their original shape as closely as possible. With the current methods where such packers are being placed in and retrieved from the tubing or wellbore by a wireline or electric line, there is a limited tensile force that can be applied to remove a packer after it has been set and released. Any deformation of the elements after they have been compressed and then released, resulting in portions of the elements sticking out radially, can cause sticking or jamming of the packer. Due to the limited tensile forces that can be put on a wireline or electric line, the packer may become stuck and require time-consuming fishing operations in order to be retrieved. Passing through other restrictions, such as subsurface safety valves, may cause them to malfunction if the packer is hung.
In the past, the tendency of the elements, particularly the upper and lower in a stack of three, to extrude has been recognized. Various techniques have been employed to minimize the extrusion tendencies of the outer sealing members in a stack of elements on a packer. Metal bands have been put on the uppermost surface of the top element and the lowermost surface of the lower element as a means for limiting the extrusion effect. Reinforcing bands, placed strategically in the uppermost corner of the upper element and likewise in the lower and outermost corner of the lower element, have also been used as extrusion prevention techniques. Typical of such techniques are U.S. Pat. Nos. 4,852,649 and 5,096,209.
The problem with these attempts to stiffen the corner of the upper and lower sealing elements, internally or externally, is that they only have met with marginal success. From the perspective of adding stiffening bands, for example, to the outer corner of the upper sealing element, the problem is that after the element is compressed the metal stiffening element does not have shape memory and remains in an extended position. Thereafter, it creates problems for removal of the packer if a wireline or electric line is used. In the past, using tubing to install or remove the packer, significant tensile forces could be applied to the packer to overcome the stiffness of the support rings which have been bent or forced into an interfering position. However, by use of an electric line or wireline, the engagement of these stiffening members with the tubing or casing could present resistance to extraction which exceeds the tensile limits of the electric line or wireline.
Another problem that has been encountered is that the deformation of the elements, particularly the upper element, occurs to such an extent and is so placed as to also offer a potential catchpoint in a narrow clearance installation, thereby impeding efforts to remove the packer after it is released. Experience has shown that prior art designs that use three symmetrical packing elements have an upper element which deforms and leaves a significant knot which sticks out significantly beyond the upper gauge ring and presents a hard catchpoint when trying to remove the packer after the elements have been released.
Accordingly, it is desirable to design packing elements that, inherently and without the aid of extraneous stiffening elements, tend to recover to their original shape as much as possible. Additionally, as disclosed in the apparatus of the present invention, it is desirable to configure the packer elements in such a way as that they have preprogrammed within them, upon assembly, built-in forces which tend to limit extrusion and further limit development of knots, which have been the experience when using symmetrically cross-sectioned elements in a stack on a packer. Accordingly, one of the objects of the present invention is to design a packing element that is so shaped so as to resist extrusion. Another object of the invention is to provide a packing element shape for an upper and lower element which take into account the uneven loading seen by those elements and due to configuration promote the ability of the upper and lower elements to contain the central element. Another object of the invention is to configure the upper and lower elements in such a way as to resist their tendency to extrude.